The present invention relates to a light beam synchronization detector for detecting the timing of a light beam spot passing through a prescribed position and a printer and relates to, for example, a light beam synchronization detector that constitutes a synchronization detection circuit used as a synchronization sensor for an optical scanning recorder.
Conventionally, a photodiode is employed as a photodetector for detecting the timing of the passing of a light beam spot in a laser beam printer or the like. That is, a photodiode is arranged in a position where the light beam spot passes, and printing start timing is detected by detecting a change in the photocurrent outputted from the photodiode.
According to this light beam detection method, the photocurrent outputted from the photodiode also changes when the intensity itself of the light beam changes, and the signal level due to this photocurrent also changes. Therefore, when comparing this signal level with a prescribed threshold value and detecting the timing of the passing of the light beam spot over the photodiode, a detection error of the timing occurs due to the change in the signal level.
In order to cancel this error, there is proposed a technique for detecting the aforementioned timing by employing a photodiode as a photodetector whose light-receiving surface is divided into two parts and taking an optical push-pull (refer to, for example, JP 04-247761 A). According to this, the detection timing can be kept constant even if the wave height of the photocurrent outputted from the photodiode changes.
It is sometimes a case where a beam spot is caused by reflected light in an optical path different from the designed prescribed optical path as a consequence of scanning the spot of a light source by means of a polygon mirror in an optical system in which the construction of the light beam synchronization detector is provided by the aforementioned technique and made incident on the photodiode. Although the quantity of light of the beam spot due to this reflected light is sufficiently smaller than the quantity of light of the light beam that should be detected, the quantity of light is disadvantageously detected by this light beam synchronization detector. When the light beam synchronization detector is built in a printer, the detection of the reflected light as described above causes the malfunction of the printer.
Accordingly, in order to prevent the erroneous detection attributed to the reflected light, there is proposed a light beam synchronization detector that does not output a detection signal when a light beam of a quantity of light being not greater than a prescribed threshold value is made incident on the photodiode.
As shown in FIG. 9, in this light beam synchronization detector, the output side of a first photodiode PD1 is connected to a first amplifier 11, and the output side of a second photodiode PD2 is connected to a second amplifier 12. This first amplifier 11 has an output terminal connected to an input terminal of a comparator 13 via a constant-voltage source 14, while the second amplifier 12 has an output terminal connected directly to the other input terminal of the comparator 13. On the other hand, the input side of the first and second photodiodes PD1 and PD2 are grounded via a constant-voltage source 10. As shown in Fig. 10A, the light-receiving surface of the first photodiode PD1 and the light-receiving surface of the second photodiode PD2 are arranged at a prescribed interval in the direction in which the light beam spot indicated by the arrow 21 advances.
In this light beam synchronization detector, the output signal of the first photodiode PD1 is convened from a signal current into a signal voltage by the first amplifier 11, inverted, amplified, further shifted in level by the constant-voltage source 14 and inputted to the comparator 13. On the other hand, the output signal of the second photodiode PD2 is converted from a signal current into a signal voltage by the second amplifier 12, inverted, amplified and inputted to the comparator 13.
The signal waveforms when the light beam spot passes in the direction indicated by the arrow 21 as shown in FIG. 10A are shown in FIG. 10B. In FIG. 10B, the waveform of a signal current 11 due to the output signal of the first photodiode PD1 is indicated by solid lines, and the waveform of a signal current 12 due to the output signal of the second photodiode PD2 is indicated by dashed lines. The waveform of a signal voltage V1 that is outputted from the first amplifier 11 and shifted in level by a threshold value Vth by the constant-voltage source 14 is indicated by solid lines. The waveform of a signal voltage V2 outputted from the second amplifier 12 is indicated by dashed lines.
The signal voltages V1 and V2 are inputted to the comparator 13, and this comparator 13 outputs an output pulse Vout1, which rises at a crossing point X1 of the signal voltage V1 and the signal voltage V2, to an output terminal OUT.
In this case, if a beam spot due to reflection is made incident on the second photodiode PD2 and a protuberance BSI is caused by the beam spot due to the reflection in the waveform of the signal current I2 as shown in FIG. 10B, then a protuberance BSV is generated in the waveform of the signal voltage V2. However, when the level of this protuberance BSV is lower than the threshold value Vth, a protuberance BSP is not generated in the waveform of the output pulse Vout1.
However, when the signal voltage outputted from the first amplifier 11 is shifted in level by the constant-voltage source 14 as in this light beam synchronization detector, another problem described as follows occurs.
That is, it is difficult to consistently keep the quantity of light of the beam spot constant, and it is required to consider a change to a certain extent. Therefore, if, for example, the light beam intensity of the light beam spot that advances in the direction indicated by the arrow 21 in FIG. 10A changes, then the crest values of the waveforms of the signal currents I1 and I2 change, and the signal voltages V1 and V2 change as indicated by one example as shown in FIG. 10C. That is, the voltage V1 changes like voltages V1a, V1band V1c, while the voltage V2 changes like voltages V2a, V2b and V2c. With the above voltage changes, the crossing point X1 of the voltages V1 and V2 changes like crossing points X1a, X1b and X1c. That is, the timing of the fall of the detection pulse Vout1 is to change within a time range of ΔX1 by the voltage changes. Therefore, a problem that an error occurs in the detection timing of the light beam spot occurs. Therefore, when this light beam synchronization detector is incorporated into, for example, a printer, the printing start timing shifts and the printed letters disadvantageously become blurred.